Hard disc drives employ a variety of means for moving the head arm assembly across the magnetic medium. Head arm assemblies typically consist of a magnetic head for reading information from or writing information to a storage disk located at one end of the arm and some mechanism for moving the arm on the opposite end. One technique for moving the head arm assembly is utilization of an arm that pivots about an axis thereby extending the heads in an arcuate fashion across the area of the surface of the disk. The rotation motion may be accomplished by utilizing a voice coil and permanent magnet arrangement. Such a device is known as a rotary voice coil motor to those familiar with industry practices and techniques.
Alternatively, the voice coil motor may be adapted to move the head arm assembly along a straight radial line with respect to the storage disk. Such devices are referred to as linear voice coil motors and are well known in the prior art. Linear voice coil motors typically include a carriage assembly which reciprocally moves along one or more guide rails. The head assembly is affixed to the carriage assembly proximately located near the storage disk. A magnetic block assembly and voice coil, often symmetrical to the longitudinal centerline of the carriage assembly, generate force for moving the carriage assembly toward or away from the storage disk.
Both linear and rotary voice coil motor construction normally comprises a loop of wire containing numerous windings physically located between one or more permanent magnets. The windings are supported by and encircle a bobbin shaped in such a manner as to cooperate with the permanent magnet dimensions. The permanent magnet(s) are attached to pole devices and the entire apparatus is physically attached to the disc drive casing at the end of the head arm assembly not containing the magnetic heads. The wire loop of the voice coil contains electrical connections which enable the coil to carry a current thereby generating a magnetic field which interacts with the field of the permanently mounted permanent magnets on the poles. Either the permanent magnet(s) and respective pole(s) or the voice coil, may be attached to the head arm assembly. Typically the voice coil is attached because of its lighter weight and corresponding lower resultant force necessary to move the head arm assembly. Although further reference throughout this document will describe rotary voice coil motor applications, similar concepts are applicable to linear voice coil motors.
The torque capability of the rotary motor is dependent upon several factors. The strength of the permanent magnet field in the voice coil region, temperature, number of windings, and material composition and uniformity all contribute to available torque. The torque generated between the conflicting magnetic field of the current carrying voice coil and the permanent magnets is proportional to the amount of electrical current in the coil and results in the head arm assembly rotating about its pivot axis.
Magnetic disk drives are becoming increasingly smaller in order to meet the portability requirements of lap-top and notebook computers. Along with the smaller drive size are the attendant problems of less space to place component assemblies such as the voice coil motor. It is therefore crucial that the flux field generated by the permanent magnet and poles of the voice coil motor provide maximum constant torque over the pole area.